Field of the Invention
The invention relates to the technical field of the treatment of dust from steelworks, notably electric steelworks.
According to a first general object, the invention discloses a method and an installation for treating said dust allowing elimination of the heavy metals it contains.
According to a second general object, the invention discloses a method and an installation for treating said dust allowing maximum economic recovery of the most abundant fraction of this dust.
The term heavy metal, which has come into use in many legislations, relates to metals which are generally:
non-degradable over time;
toxic to living systems at very low concentrations;
inclined to accumulate in living organisms and to become concentrated in the course of transfers of materials in food chains.
Steelworks dust, for example issuing from remelting in the electric furnace of scrap iron for the production of steel, contains such heavy metals such as zinc, cadmium and lead, for example.
Dust production is between 15 and 22 kg per tonne of liquid steel.
As a guide, steel production, in Europe, was of the order of over 500,000 tonnes per year at the beginning of the 1990s.
Consequently, there is a considerable need for effective treatment of this dust, for the sake of protecting the environment notably.
A number of methods have been envisaged for the treatment of this dust, the methods varying depending on the composition of said dust.
Two main classes of dust are distinguished, depending on the nature of the steels produced, carbon steels and stainless alloyed steels.
In the case of carbon steels, zinc (17 to 29%) is present in the dust in two forms: zinc oxide, ZnO, and zinc ferrite, ZnFe2O4, while the lead (4 to 5%) is in oxide form (PbO) The cadmium contents are lower, of the order of around 800 ppm.
Unlike zinc oxide, zinc ferrites are not easily soluble during hydrometallurgical treatments for releasing the zinc.
For alloyed and stainless steels, the quantity of dust generated in Europe was around 50,000 tonnes per year during the 1990s.
Various procedures and processes, many still at the experimental laboratory stage, have been devised for the treatment of dust from steelworks, notably electric steelworks.
The BUS (BERZELIUS UMWELT) procedure proposes a treatment of dust from steelworks for carbon steels combining two processes: the WAELZ process and the ISP (IMPERIAL SMELTING PROCESS). This procedure uses a conventional pyrometallurgical process and condensation of the zinc vapours by means of liquid lead spray condensers.
The FLAME REACTOR process, used for the treatment of dust from alloyed and stainless steels, consists essentially of vertical flame smelting/reduction cyclone separation. The waste products, rich in zinc and lead, are converted into slag to be resmelted and into oxide to be recovered.
The PLASMADUST process has been used since the 1980s for the treatment of electric steelworks dust rich in zinc and lead. The treatment furnace is of the tank furnace type, whose energy is provided by blown arc plasma torches. The energy of the plasma is used to heat the reagents, and to provide the heat necessary for the endothermic reduction reactions. The plasma-producing gas is introduced into the furnace by water-cooled copper nozzles. The powdery charge composed of dust, flux and coal is injected into the nozzles and mixed with the plasma-producing gas. The fumes charged with metallic vapours leave the furnace at a temperature of around 1150xc2x0 C.
When the PLASMADUST process is used for dust from alloyed and stainless steels, the zinc content in the fumes is generally too small for condensation of the zinc to be efficient, the zinc in this case being retrieved from the sludge issuing from the washers.
The PLASMINOX process uses a treatment by means of hot cathode plasma torches operating by transferred arc at D.C. and installed vertically on the furnace. In the furnace, the metals presentxe2x80x94chromium, nickel, molybdenum and ironxe2x80x94are separated from the dust by melting and the ferroalloy which results therefrom is poured into moulds for subsequent recharging at the dust-producing steelworks. The scorias obtained are, in accordance with the standards, inert. The fumes escaping from the furnace are cooled and have their dust removed. The secondary dust obtained contains volatile metals Zn, Pb which are in the form of oxides and are not recovered.
The modified ZINCEX process produces zinc by hydrometallurgical means, this process having the following successive steps:
atmospheric leaching of the dust by a dilute sulphuric acid solution;
purification of the leaching solution by precipitation of the iron and aluminium;
selective extraction of the zinc by D2EHPA;
stripping of the zinc from the organic phase by the highly acid electrolytic solution;
electrodeposition of the zinc on to aluminium cathodes.
The ZINCEX process is relatively complex in its implementation since it requires organic solvents combined with sulphuric acid. Furthermore, this process requires large quantities of wash water to avoid the formation of SO2 from the oxides which are recycled in the furnace. This ZINCEX process is not easily exportable, owing to the above-mentioned problems and since it necessitates too heavy investments.
The GLASSIFICATION process comprises a step of mixing the steelworks dust with other steelwork waste products and constituents of glass. The mixture is melted in a submerged arc furnace. The metals vaporized during the melting condense in the upper part of the furnace before being trapped in the melt. The glass obtained can be put in the form of granules for the manufacture of glass tiles or grit.
In total, three main ways of recycling have been envisaged for recycling dust from electric steelworks.
A first way consists of performing reductions at various temperatures in accordance with the processes for vaporizing the heavy metals and re-injecting the ferrous fraction into the steel production furnaces:
the document EP 336 923 proposes treating steelworks dust in an iron smelting furnace after conversion into pellets;
the document EP 441 052 proposes a thermal treatment between 1200 and 1700xc2x0 C. by the addition of reducing agents;
the document WO 91/12 210 describes a method of treating steelworks dust in an iron reduction furnace, the heavy metals being retrieved by condensation from the hot gases;
the document EP 453 151 describes a method of treating dust in the form of pellets by an agent selectively reducing iron oxide;
the document FR 2 666 592 describes a device for extracting volatile metals acting by oxidation;
the document WO 93/69 619 describes an arc furnace specially designed for the reduction of dust containing oxides;
the document EP 551 992 describes a method of retrieving recoverable metals from steelworks dust by reduction and vaporization of the recoverable metals;
the document FR 2 373 612 describes a method of extracting zinc contained in steelworks dust, by selective oxidation under heat.
A second way consists of directing the dust towards a use where it would be xe2x80x9cmade inertxe2x80x9d by various means with a view to use as construction or filler materials:
the document EP 402 746 proposes recycling by incorporation in a clay for a mine packing material;
the document WO 91/12 210 proposes the use of steelworks dust for the treatment of sewage with the addition of a flocculating agent;
the document FR 2 689 881 describes a method of manufacturing bricks whose properties are improved by the addition of electric steelworks dust first calcined at the vaporization temperature of heavy metals contained in said dust;
the document FR 2 700 161 describes a mixture for a road surfacing comprising 2 to 6% by weight of electric steelworks filter dust.
Pyrometallurgical processes, with or without electricity, allow the recovery of oxides of zinc, of iron or nickel chromium alloy or of liquid zinc most often by condensation.
A third way, hydrometallurgical, has also been envisaged.
Various media such as sulphuric acid, hydrochloric acid, soda and ammonia have been tried out.
The document FR 2 716 895 describes a method of treating electric steelworks dust with or without prior reduction, or electrolytic galvanizing dust. Said method comprises an ammoniacal attack by an ammoniacal buffer solution, then a desorption of the ammonia by air flow cleaning.
Direct attacks in a sulphuric or hydrochloric medium are not economically viable, since the volumes in terms of iron hydroxide or salts involved are enormous.
These residues are moreover still contaminated by metallic impurities such as lead and cadmium.
Alkaline leaching has been proposed.
The document FR 2 535 736 describes a method of treating dust containing zinc, issuing from electric steelwork furnaces, in which the dust is subjected to a basic leaching. The basic rejects resulting from the different washing phases and containing zinc are neutralized by means of the acid rejects coming from galvanization operations and also containing zinc.
The document FR 2 501 141 describes a basic leaching of zinc-bearing dust by means of ammonium chloride, the powder obtained being agglomerated and introduced into a smelting furnace.
By the leaching technique, only the elements in the form of free oxides (mainly zinc and lead) are dissolved. The mixed oxides of non-ferrous metals/ferrous oxides are not affected. Their management is therefore problematic, including re-injection into the steelworks furnace which leads, in the end, to a problem of re-concentration of impurities.
The methods combining liquid ammonia with ammonium salts (notably the ENGITEC process described in European patent application EP-93 00018.5) have drawbacks in the handling of chlorides for the retrieval of minority elements (zinc, lead and cadmium) and the recovery of iron oxides. The high chloride content prevents recycling of these phases in the furnaces owing to the problems related to chlorine. Their washing would require considerable volumes of water.
It can therefore be thought that this type of method has a maximum efficiency between 25 and 30%, proportional to the concentration in the non-ferrous phase. This method therefore generates a waste product whose management remains as problematic as that of the untreated dust.
The document FR 2 737 503 describes a method of preparing mineral pigments, and the mineral pigments thus obtained, and an installation for the implementation of such a method. This document, coming from the applicant, describes a method having the following steps:
separation of the dust into two fractions, one fraction comprising magnetic elements and one fraction comprising non-magnetic elements;
basic leaching of the non-magnetic fraction in order to dissolve the zinc fraction which would not be bound up in spinel form, and the silica, lead, and a fraction of the manganese;
rinsing until neutralized and separation of the charge thus obtained;
calcination at a temperature between 450 and 650xc2x0 C. of the charge thus obtained;
treatment of the calcined charge with sulphuric acid in the presence of a catalyst for solubilizing the iron oxides formed during the calcination step and the heavy metals other than zinc;
collection of the mineral pigments;
use of the solutions issuing from the rinsing and the sulphuric acid treatment for precipitating other pigments.
The present invention proposes a method of neutralizing the free heavy metals contained in the dust from steelworks, notably electric steelworks, said method also allowing the recovery of the most abundant fraction of said dust.
The method according to the invention makes it possible to treat indiscriminately the so-called acid dust and the so-called basic dust.
The method according to the invention makes it possible to geographically dissociate pre-treatment of the dust at the collection site and chemical treatment and recovery from said dust.
The method according to the invention is particularly adapted to the treatment of dust from carbon steels and tool steels.
The invention relates, according to a first aspect, to a method of treating steelworks dust with a view to retrieval of the recoverable elements, said method comprising an attrition in water followed by a water grading of the charge thus obtained, said method being characterised in that it also comprises:
washing of the charge collected at the overflow issuing from the water grading, said charge being charged with dissolved heavy metals and salts, with a view to separating the water-soluble saline fractions from the insoluble oxides,
treatment under heat of the whole of the charge, washed in the previous washing step without prior magnetic separation, with a view to eliminating the metals in the form of free oxides such as zinc and lead,
treatment by thermal means of the charge obtained after treatment under heat at a temperature between 240 and 800xc2x0 C.
The method also has the following aspects, possibly combined.
A step of treatment with sulphuric acid with a concentration of between 5 and 8% is carried out after the treatment by thermal means.
The treatment under heat performed after the washing step is, according to a first variant, carried out in an acid medium. Said acid medium is, in one embodiment, a sulphuric acid solution at a concentration of between 8 and 18% and a temperature of between 40 and 95xc2x0 C.
The treatment under heat is followed by an oxidation and then a separation of the iron in the form of an insoluble oxide precipitate.
The zinc is separated by means of an acid electrolysis.
The treatment under heat performed after the washing step is, according to a second variant, carried out in an alkaline medium.
According to one embodiment, the washing comprises treatment with a sulphur derivative chosen from amongst the group of heterocyclic compounds such as trimethyl triazine or 2,5 dimercapto thiadiazole.
The alkaline treatment is performed in a concentrated soda solution under heat in the presence of an oxidizing agent.
The concentration of the soda solution is between 240 and 400 g/l.
The temperature of the alkaline treatment is between 50 and 110xc2x0 C.
The oxidizing agent is chosen from amongst the compounds which are oxidizing in an alkaline medium such as hydrogen peroxide.
The hydrogen peroxide concentration is between 10 and 40%, and more particularly between 30 and 35%. The lead contained in the alkaline solution is eliminated by means of hydrogen peroxide with a concentration equal to at least 35% by weight.
The treatment by thermal means is performed between 400 and 650xc2x0 C.
The treatment with sulphuric acid is carried out in the presence of an injection of air in order to oxidize the divalent iron into trivalent iron.
The treatment with sulphuric acid is carried out under heat at a temperature between 40 and 80xc2x0 C.
The solution issuing from the acid attack is adjusted to a pH between 5.4 and 10.
The solution issuing from the acid attack is adjusted to a temperature between 25 and 80xc2x0 C.
The solution issuing from the acid treatment is purified by means of iron or zinc powders.
The method also comprises a two-stage electrolysis of the solution issuing from the alkaline treatment.
Said first electrolysis uses a cathode of graphite in grains contained in a basket in contact with a titanium grid.
The basket is made of polypropylene.
The anode is made of titanium containing ruthenium or iridium.
The second electrolysis comprises an anodic re-dissolution of the graphite cathodes in a sodium sulphate solution and a cathodic re-deposition of the zinc on an iron or steel electrode.
The second electrolysis is carried out in an electrolyser whose pH is adjusted to a value close to 5.
Other objects and advantages of the invention will emerge in the course of the following description of embodiments, said description being given with reference to the accompanying figures in which: